He’s a first-time candidate who claimed the biggest upset of the election season. But newcomer Seth Moulton was not name-checked by Governor Deval Patrick or Mayor Martin J. Walsh at the Democrats’ post-primary celebration Wednesday morning. Instead, Moulton stood mostly unnoticed amid a crowd of candidates who had lost.

It was an hour and 45 minutes when the host finally introduced the guy who had snatched the nomination from fellow Democrat, US Representative John F. Tierney.

Still, Moulton thanked the crowd “for being so welcoming to me here this morning.” It was the first time he had received an invitation to a Democratic party event.

Advertisement

Moulton, 35, a graduate of Harvard’s Kennedy School of Government and a Marine veteran who did four tours in Iraq, upended expectations of the Sixth Congressional District race Tuesday night, when he muscled an 18-year incumbent out of his seat and sought to claim the role of change agent. But there already was a candidate playing that role: Republican Richard Tisei, who had come within 4,000 votes of beating Tierney two years ago and was set to launch a rematch.

So the candidates immediately began repositioning themselves Wednesday. Tisei, who served 26 years in the Legislature, emphasized his experience and his work across the aisle, painting Moulton as a johnny-come-lately who will vote just like the last Democrat did. “I won’t need to go to Harvard Kennedy School to learn how to be a legislator,” Tisei told reporters at his campaign headquarters Wednesday. “I know how to do that now.”

Moulton painted Tisei as a creature of Beacon Hill, a twist on the usual script.

With his overwhelming victory, the Sixth Congressional District voters “clearly showed that they have a mandate for new leadership, and new leadership is not a career politician who was first elected to office when I was 6 years old,” Moulton told reporters Wednesday at the Democratic unity breakfast. The morning-after ritual brings together the winners and losers from Tuesday’s primary to rally against their common Republican foes.

Advertisement

Two years ago, Tisei’s party registration seemed to be the biggest problem standing between him and election. The former state Senate minority leader, who ran on a ticket with gubernatorial nominee Charlie Baker in 2010, is a progressive Republican who is openly gay. Nonetheless, he was caricatured by national Democrats as “Tea Party Tisei,” in his race two years ago.

The Democratic Congressional Campaign Committee focused its pro-Tierney efforts that year on the message that regardless of his personal political beliefs, if Tisei were sent to Washington, he would vote with the national Republican leadership, which is unpopular in Massachusetts.

This time around, Tisei has taken more of the lead in defining himself. On Wednesday, a day after he was nominated without opposition, he launched a television ad reintroducing himself to voters as a “bipartisan leader” and “voice of moderation.” (The ad was planned, regardless of the Democratic primary outcome, an aide said.)

“I don’t understand why people can’t put party aside and do what’s right for the country,” Tisei says in the ad.

Less than an hour after Moulton’s win Tuesday night, The National Republican Congressional Committee came out swinging, panning him as “a less effective carbon copy” of Tierney in a press release Tuesday night. And the group tried to use the shifting sentiments in Washington to their advantage: With Democrats in the minority, they suggested, Tisei would be better positioned as a Republican to protect Massachusetts interests.

Advertisement

“Given that Republicans will hold the House and seem poised to capture the Senate, it becomes even more important to have someone like Tisei in the majority working on behalf of Massachusetts families and small businesses,” NRCC spokesman Ian Prior said in a statement Wednesday.

Tisei himself said he was untroubled by the shift in Democratic protagonists. But national political observers had considered Tisei a stronger contender against Tierney, who had been vulnerable since 2010. That year, his wife was convicted on federal tax charges for her involvement with her brother’s finances, in connection with the family’s offshore gambling operation. Though he was not implicated, Tierney had been eyed warily by voters after professing ignorance about the income brought in by his wife.

While the Republicans went after Tierney in previous campaigns, during this primary season, Moulton mostly steered clear of the scandal.

Tierney, meanwhile, tried to paint Moulton as a closet Republican, airing a negative television ad that cast a jaundiced eye on his campaign contributions from some people who also donate to Republicans.

Moulton said Wednesday that he did not feel the Tierney ad hurt voters’ impressions of him, although he immediately faced skepticism from one Democrat at the unity breakfast who said Moulton would need to start “convincing me that you were a real progressive” before he would volunteer.

“I think most voters realized that it just wasn’t true,” Moulton said of Tierney’s negative ad. “In fact, I think some of the negative backlash from that ad actually helped my campaign.”

Advertisement

And what about his opponent? Moulton was asked by a reporter. Does Moulton think of him as “Tea Party Tisei?”

“I think Richard Tisei is a Republican,” Moulton said. “And I’m going to talk with voters about how my Democratic values differ from his Republican values.”